feat(local): report unclassified and dependency-blocked failures for discovery (#122)

## Why

Issue #120 (the auto-proposed-patches issue) only lists **auto-proposable** fixes -- currently just the TBB signatures. So a reasonable read of it was "TBB is our only build failure", when in fact three whole categories are simply not shown there:

- **Unclassified failures** -- anything that doesn't match a seeded signature is routed to human triage and never appears (we've only seeded TBB and libuv signatures).
- **Dependency-blocked failures** -- the ~800 RcppParallel dependents (post #121) are still failing; they only show as a log line.
- Human-only signatures (libuv).

These blind spots are exactly where the *next* signatures should come from, so they deserve the same visibility as the proposals.

## What

Extend the feedback-loop tracker to surface the classifier's blind spots:

- **`unclassified_summary()`** -- groups every unknown-signature failure by normalised fingerprint, ranked by build count, capped with an explicit `dropped_groups` count (no silent truncation), each with example packages + platforms. These are the candidates for new `build_signatures()` rules.
- **`blocked_summary()`** -- lists each dependency (e.g. RcppParallel) and how many dependents wait on it.
- `proposal-tracking.R` prints both sections, and a new **`--open-issue`** mode posts/updates a *"Unclassified build failures (needs signatures) (#115)"* Forgejo issue.
- The weekly crow pipeline now runs the tracker with `--open-issue`, so it maintains a second tracking issue alongside the proposals one. Read-only on the DB; the only writes are the two issues.

## Verification

- New tests cover `unclassified_summary` (ranking + both caps) and `blocked_summary`.
- Tracker smoke with a stubbed DB (proposable + blocked + unclassified mix) prints the hit rate, `Blocked on a dependency: RcppParallel: 2 dependent(s)`, and `Unclassified failures ... [2 builds | 2 pkgs] ld: undefined reference ...`.
- Full suite: 95 tests pass; all pre-commit hooks pass (air, prettier, markdownlint, yamllint, validate-patches).

Reviewed-on: #122

.crow/weekly-patch-proposals.yaml

@@ -1,12 +1,13 @@
 # Weekly failure-triage proposals (issue #115, steps 3 + 4).
 # Classifies the recorded `single_builds` failures and:
-#   1. posts/updates a Forgejo tracking issue with the auto-proposable registry
-#      entries (human reviews, trial-builds, and opens the PR -- nothing merges), and
-#   2. prints the feedback-loop metrics (signature hit rate, proposed-vs-merged,
-#      retirement candidates) to the run log.
+#   1. posts/updates a Forgejo issue with the auto-proposable registry entries
+#      (human reviews, trial-builds, and opens the PR -- nothing merges),
+#   2. posts/updates a second Forgejo issue with the classifier's blind spots
+#      (unclassified failures = candidates for new signatures, and groups
+#      blocked on a dependency), and prints the feedback-loop metrics to the log.
 # Global across platforms (the classifier groups over all of single_builds), so
-# a single job -- no matrix. Clones read-only; the only write is the Forgejo
-# issue via FORGEJO_TOKEN.
+# a single job -- no matrix. Clones read-only; the only writes are the two
+# Forgejo issues via FORGEJO_TOKEN.
 when:
   - event: manual
   - event: cron
@@ -36,7 +37,7 @@ steps:
       - rm -rf /mnt/cache/R-pkgs/00LOCK-*
       - /opt/R/$R_VERSION/bin/R -q -e 'pak::pak(c("httr2", "jsonlite"))'
       - /opt/R/$R_VERSION/bin/Rscript local/propose-patches.R --open-issue
-      - /opt/R/$R_VERSION/bin/Rscript local/proposal-tracking.R
+      - /opt/R/$R_VERSION/bin/Rscript local/proposal-tracking.R --open-issue
     backend_options:
       kubernetes:
         resources: